Preserve security key signature blobs

[nataphon-ktsystems]

Security-key (sk-*) signatures carry a structured OpenSSH signature blob: algorithm, raw signature, flags, and counter. The client-side auth path and OpenSSH agent proxy path were treating those like ordinary two-field signatures, which dropped or double-wrapped the trailing security-key fields.

This change preserves the full security-key signature blob when reading OpenSSH agent responses, forwarding local agent responses, and appending the final userauth signature. Ordinary key types keep the existing algorithm-plus-signature wrapping.

Tests cover both sides of the client path:

• AgentUnitTest.securityKeySignatureBlob verifies a security-key signature blob survives the agent protocol path with flags and counter intact.
• UserAuthPublicKeySkTest.securityKeySignatureBlobIsNotWrapped verifies the client userauth packet writes the security-key signature blob directly instead of wrapping it again.

Validation:

./mvnw -pl sshd-core -am -Dtest=AgentUnitTest,UserAuthPublicKeySkTest -Dsurefire.failIfNoSpecifiedTests=false test

[tomaswolf]

Thanks a lot. I've added an integration test in a follow-up commit.

[nataphon-ktsystems]

Glad it helps! Thanks for such a responsive review. Looking forward to be the part of Apache community.